1. Field of the Invention
This invention relates to wireless communication systems and, more particularly, to a wireless communication system using modulated backscatter technology.
2. Description of the Related Art
Radio Frequency Identification (RFID) systems are used for identification and/or tracking of equipment, inventory, or living things. RFID systems are radio communication systems that communicate between a radio transceiver, called an Interrogator, and a number of inexpensive devices called Tags. In RFID systems, the Interrogator communicates to the Tags using modulated radio signals, and the Tags respond with modulated radio signals. After transmitting a message to the Tag (called the Downlink), the Interrogator then transmits a Continuous-Wave (CW) radio signal to the Tag. The Tag modulates the CW signal using Modulated BackScattering (MBS) where the antenna is electrically switched, by the modulating signal, from being an absorber of RF radiation to being a reflector of RF radiation. This modulated backscatter allows communications from the Tag back to the Interrogator (called the Uplink). Conventional MBS systems are designed a) to identify an object passing into range of the Interrogator, and b) to store data onto the Tag and then retrieve that data from the Tag at a later time in order to manage inventory or perform another useful application.
As mentioned above, an RFID Tag can be used to identify an object in inventory. One of the problems in inventory applications is that, to identify the item, you must first know where the item is located. For example, using optically scanned bar codes, the optical bar code scanner must be very near the item in order to scan the bar code. RFID technology has improved this situation somewhat, in the sense that a greater range can be supported between the Tag and the Interrogator. However, RFID technology has range limitations, and the effective range of the best RFID technology is still small compared to the physical dimensions of a large warehouse.
Therefore, we consider the application of RFID technology to the problem of determining the location of an RFID Tag.